Electropolishing with phosphorous acid



* Uni-ted t msPa eh Chevy Chase, Md., assignors to the United -1States of America, as represented by the Secretary of Commerce No Drawing. Application April 1, 1953,

Serial N0. 346,294

1 Claims. (21. 204-140; V (Granted under Title 35, U. s. Code 1952 see. 266) The invention described herein may be manufacturedand used by or for the Government of the United States for governmental purposes without the payment to us of any royalty thereon in accordance with the provisions of 35 U. S. Code (1952), section 266.

The present invention relates to a method and bath for electropolishing a number of metals, and in particular to electropolishing with a phosphorous acid bath.

It is an object of the present invention to provide a bath for electropolishing a number of difierent metals.

Another object of the present invention is to provide a bath for electropolishing a number of difierent metals, which bath can be used at relatively low temperatures.

Another object of the present invention is to provide a method of electropolishing a number of different metals in which phosphorous acid is used as the main constituent of the bath.

In accordance with the present invention the metal to be polished is connected in the electrical circuit as the anode. The cathode of the circuit may be carbon or any other conductive material which is not attacked by the bath. The current densities may range from to 100 amperes/decimeter and the temperature may vary from 20 to 100 degrees centigrade. The particular current density and temperature depends upon the material being.

polished and the make-up of the bath. The main ingredient of the bath is phosphorous acid, which is available commercially in crystalline 100 percent acid or in 75 percent solution. The crystalline acid melts at 74 degrees centigrade and can be used in the fused state or can be diluted with water or mixed with other acids or organic compounds. A bath which was found to be suitable for polishing copper, brass, nickel, aluminum, cadmium, and stainless steel, was composed of 90 percent by weight of phosphorous acid and percent by weight of water. The acid concentration is not too critical. Electropolishing will take place with concentrations as low as 60 percent.

Solution No. 4 (see Table 2) which contains phosphorous acid, 85-90 percent by weight, water, 0-l0 percent, and hydrofluoric acid, 0-5 percent, will polish copper, brass, silver, and molybdenum. This solution will also polish zinc if the hydrofluoric acid concentration is kept at about 2 percent. Higher concentrations of hydrofluoric acid cause a black surface on the zinc.

Although there are many variations possible in the preparation of phosphorous acid electropolishing solutions, the two that seem the most promising are solutions No. 2 and No. 4. Solution No. 4 is made by adding a small amount of hydrofluoric acid to solution No. 2. Solution No. 4, although not usable with all of the metals which solution No. 2 will polish, does polish some metals which solution No. 2 will not polish, and extends the range of metals which can be polished with a bath having phosphorous acid as the basic ingredient.

When aluminum is polished in the 90 percent phosphorous acid bath, solution No. 2, a thick oxide film forms on the surface. This film, which appears as a slight bloom 2,740,755 Patented Apr. 3, 19 56 on the surface, can .be removed by dipping in dilute sodium hydroxide, leaving a bright surface on the aluminum. 1

:Set outbelow is a list of metals and thesolutions used to. electropolish them.

Table 1 Poltsh- Current Temperamtal ing Density ture Time Solution (amp. (degrees (min) No. dm!) 0.)

2 .10-30 '70 2-10 3 50-100 20-60 2-5 ,5 .10-40 room 2-15 ge 2 p 10-40 30-50 '2-10 o 4 10-40 30-70 2-10 2 10-40 30-90 2-10 4 10-40 30-70 2-10 5 10-40 room 2-15 2 30-00 4 10-40 room 2-15 4 10-40 room 2-15 1 10-40 80 2-15 4 10-40 room 2-15 2 10-40 room 2-15 2 30-40 30-50 2-15 The following solutions were typical baths used for electropolishing.

Table 2 Solution No H PO H o 2$? Ethyl cerin Alcohol 1 percent by weight" 100 2 do 90 1o 60 40 -90 0-10 0-5 so 10 10 While the invention has been particularly described in connection with several specific bath compositions,it will be understood that various combinations and concentrations of the constituents mentioned, as well as various operating conditions might be employed within the scope of the invention, and that various modifications can be made within the scope of our invention as defined in the appended claims.

We claim:

l. The method of electropolishing various metals selected from the group consisting of aluminum, copper, brass, stainless steel, zinc, silver, molybdenum, cadmium and nickel which comprises connecting the metal to be polished as the anode in a bath consisting essentially of between 60 to 100 percent by weight of phosphorous acid, and passing an electric current of from 5 to 100 amperes/decimeter between said anode and a cathode.

2. The method of electropolishing various metals selected from the group consisting of aluminum, copper, brass, stainless steel, zinc, silver, molybdenum, cadmium and nickel comprising connecting the metal to be polished as the anode in a bath which consists essentially of at least.60 percent by weight of phosphorous acid, and passing an electric current of from 5-100 amperes/decimeter between the anode and a cathode, said bath being maintained at between 20 to 100 degrees centigrade.

3. The method of electropolishing molybdenum which comprises connecting the metal to be polished as the anode in a bath consisting of melted 100 percent phosphorous acid and passing an electric current of between 5 to 100 amperes/decimeter between the anode and a cathode.

4. The method of electropolishing stainless steel which comprises connecting the metal to be polished as the anode in a bath consisting of percent by weight of phosphorous acid and 10 percent by weight of water, and

3 r passing an electric current of between to 40 amperes/ decimeter between the anode and a cathode, said bath being maintained at a temperature between and 90 degrees centigrade.

5. The method of electropolishing aluminum' which comprises connecting the metal to be polished as the anode in a bath consisting of 60 percent by weight of phosphorous acid and 40 percent by weight of glycerin, and passing an electric current of from to 100 ampere's/ decimeter between the anode and a cathode, the bath being maintained at temperatures between 20 and degrees centigrade.

6. The method of electropolishing silver, which comprises connecting the metal to be polished as the anode in a bath consisting of to percent by weight of phosphorous acid, 0 to 10 percent by weight of water, and 0 to 5 percent by weight of hydrofluoric acid, and passing an electric current of from 50 to amperes/ decirneter between the anode and a cathode, the bath i being maintained at a temperature between 30 and 70 degrees centigrade.

10 at approximately room temperature. 7

References Cited in the tile of this patent UNITED STATES PATENTS Faust Jan. 9, 1945 I I FOREIGN PATENTS 619,336 Great Britain Mar. 8, 1949 

1. THE METHOD OF ELECTROPOLISHING VARIOUS METALS SELECTED FROM THE GROUP CONSISTING OF ALUMINUM, COPPER, BRASS, STAINLESS STEEL, ZINC, SILVER, MOLYBDENUM, CADMIUM AND NICKEL WHICH COMPRISES CONNECTING THE METAL TO BE POLISHED AS THE ANODE IN A BATH CONSISTING ESSENTIALLY OF BETWEEN 60 TO 100 PERCENT BY WEIGHT OF PHOSPHOROUS ACID, AND PASSING AN ELECTRIC CURRENT OF FROM 5 TO 100 AMPERES/DECIMETER2 BETWEEN SAID ANODE AND A CATHODE. 